Technological development and increased demand for mobile devices have led to rapid increase in the demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries having high energy density, high operating voltage, long cycle span and low self-discharge rate are commercially available and widely used.
In addition, increased interest in environmental issues has recently brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main causes of air pollution. Such electric vehicles generally use nickel-metal hydride (Ni-MH) secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries having high energy density, high discharge voltage and stable output is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, output high power in a short time and be operable under severe conditions for 10 years or longer, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries. In addition, lithium secondary batteries used for electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like should have high rate and power characteristics according to operational conditions of the vehicles and exhibit excellent operability even at low temperatures.
Conventional lithium secondary batteries used for small-size batteries generally use lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, lithium cobalt composite oxide is disadvantageous in that cobalt used as a main element is very expensive and lithium cobalt composite oxide is unsuitable for use in an electric vehicle in terms of stability. Accordingly, lithium manganese composite oxide having a spinel crystal structure composed of manganese, which is low-cost and exhibits superior stability, may be suitable for the cathode of lithium ion batteries for electric vehicles.
However, when lithium manganese composite oxide is stored at high temperature, manganese is eluted into an electrolyte, deteriorating battery properties. Thus, there is a need for measures to prevent this phenomenon. Further, lithium manganese composite oxide disadvantageously has a small capacity per unit battery weight, as compared to conventional lithium cobalt composite oxides or lithium nickel composite oxides, thus limiting an increase in capacity per unit battery weight. Battery design to overcome this limitation should be carried out to put secondary batteries employing the material into practical use as power sources of electric vehicles.
To solve these disadvantages, studies have been conducted into fabrication of an electrode using a mixed cathode active material. For example, Japanese Patent Application Publication Nos. 2002-110253 and 2004-134245 describe technologies in which a mixture of lithium manganese composite oxide and lithium nickel cobalt manganese composite oxide is used to increase regenerative power or the like. However, these technologies still have disadvantages of poor cycle life of lithium manganese oxide and limitation of improvement in stability.
Further, Korean Patent No. 0458584 describes a technology associated with a cathode active material composed of a nickel-based large-diameter active material compound with an average diameter of 7 to 25 μm and a small-diameter active material compound with an average diameter of 2 to 6 μm (for example, LIxMn2O4-zXz, where X represents F, S or P and 0.90≦x≦1.1 and 0≦z≦0.5) to increase the bulk density of an electrode plate and thereby improve battery capacity.
In addition, to improve battery capacity, life and high-rate discharge properties, Korean Patent No. 0570417 uses lithium dimanganese tetroxide having a spinel crystal structure as a cathode active material, Japanese Patent Application Publication No. 2002-0080448 uses a cathode active material containing lithium manganese composite oxide, and Japanese Patent Application Publication No. 2004-134245 uses a cathode active material containing lithium transition metal composite oxide and lithium manganese composite oxide having a spinel crystal structure to fabricate a secondary battery.
However, secondary batteries with desired stability and life characteristics have yet to be developed.